Before any QC measures on steel billets, we try to rigorously select our vendors for the same. For us it is important that our vendors use high quality raw material in electric arc furnace, so as to billets have lower Sulphur and phosphorous impurities and better microstructure. Steel Billet vendors shall perform ultrasonic tests to detect any internal defects and report the same. Our QC team receives 3 samples from each purchased steel billet batch, which is normally 50 tones, to ensure its quality prior to delivery. Most importantly, material composition shall be in accordance with the requirement. Homogeneity of the batch will also be checked by comparing the tolerances of each element in the three samples. After peeling and removal of oxide and dust layer for visual inspection to detect any possible cracks or surface defects, a piece of a random billet goes through spectrographic quantometer for elemental analysis. Sample’s surface material is vaporized with the testing probe by an arc spark discharge. The atoms and ions contained in the atomic vapor are detected base on emission of radiation they create. Our lab team calibrate our quantometers regularly with CRM samples to avoid any misreading. In the end, one representative sample will be kept in our warehouse for six months.
Grinding Media Quality Control Measures
Quality control (QC) is our key to success. Grinding media quality affects the whole milling operations and significantly controls its costs, because grinding media quality controls grinding balls consumption rate, grinding efficiency and power consumption. Forged steel grinding balls or grinding rods quality depend on several factors. The most important one is quality of raw material, i.e. hot rolled round bars. Round bars quality is itself a function of steel billets quality. Since we produce round bars from billets ourselves, even the smallest quality issues at any step can destroy a whole batch of final product. Consequently, in Alpha Grinding Media, a robust quality control program takes place according to pre-determined procedures that will not be sacrificed at any costs. Such steps are designed to ensure faultless delivery of your orders.
Steel Billets Quality Control
Hot Rolled Round Bars Quality Control
Afterwards, in the process of production of hot rolled round bars, a series of non-destructive tests will be carried out after cutting the rods. One very delicate step is straightening which is carried out when either round bars are produced for grinding media production or requested by the client. In summary, our QC team carry out size inspections, control surface defects by Eddy Current Testing, measure round bar dimension in the stream of the mill by optical system, and detect dimensional drift out of tolerance by special lab equipment. Again a sample from each batch will be kept for future reference. As soon as quality control steps are completed, round bars get packed and are ready for shipping.
Grinding Media Quality Control
Inappropriate chemical composition, low hardness and bad microstructure among other factors can cause grinding balls failure. Poor quality control may let broken balls, shelling, capping, surface spalling and misshapen balls. High wear occurs due to incorrect hardness and microstructure. In summary, our quality control team take below measures to ensure correct supply of our steel balls. From each production batch, we take five samples of grinding balls from five different big bags to perform the following tests:
- Chemical composition analysis: Elemental analysis on steel billets is an important item in QC, because the quality of the forthcoming product is completely related to the quality of the billet. All of the elements of the alloy should be in the standard range and also the dispersity of the elements should be homogenous across the billet.
- Surface quality inspection: if steel grinding balls have any spalling or are not fully spherical in shape, they may cause damage to SAG mill or ball mill liners. In order to avoid such balls to reaching our customers, we make visual surface inspection in three steps to ensure steel balls’ shape and surface finish while minimizing flaws, grooves and folds. First, when the balls are hot right after roll forging or stamp forging or casting. Second, when the balls are cold after heat treatment. And finally prior to shipment, i.e. explained in item 8 below.
- Size/Weight controls: to ensure that any batch does not exceed the size tolerance limit, during production diameter controls are done continuously. Also the weight of the batch and some individual balls, has to be checked.
- Grinding balls / rods surface hardness test: grinding media hardness directly determines wear rate. At Alpha, we analyze surface hardness of our grinding balls by Rockwell hardness tester in our grinding media lab. If two or more of the 5 selected samples do not pass the surface hardness test, the whole batch will be categorized as lower hardness grinding balls.
- Volumetric hardness test: volumetric hardness is calculated after grinding balls are cut in halves by wire cutting. It correlates hardness at surface, ¾ radius, ½ radius, ¼ radius and center of balls. Such accurate hardness profile shows wear performance and stress level of the balls.
- Impact toughness test: free 8 meters drop test is performed to assess the durability of grinding balls against sudden impact. Grinding balls are tested against the floor material that higher in hardness comparing the balls. Floor plate hardness is determined based on the application of grinding balls.
- Abrasion/Impact wear test: for customers with special use of the balls, we could also perform wear tests on the products, which gives information on the wear rates of the material under abrasion and the impacts in the ball-mill.
- Metallographic structure analysis: metallographical analysis is performed to check the following items: 1- the microstructure of the surface and the presence of fine tempered martensite. 2- the microstructure of the center and the presence of ductile phases like pearlite and bainite. 3- the transition of the phases alongside the radius from surface to center. 4- percentage of martensite and retained austenite in the centeral parts of the ball.
- Pre-shipment visual inspection: after grinding balls are approved in the factory, they will be transferred to the warehouse, where all the balls are unloaded on a conveying table and checked one by one not to let any misshapen balls or the ones with even smallest spalling out. Once steel grinding balls / rods pass these steps, we rest assured that our customer(s) will enjoy a consistent high quality that saves a huge deal of milling consumables. We believe quality is our best advertisement that has already brought us several new customers.
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